The upper 100mm round steel post, being lowered into the top of the 150 base support, At the top of the 150 support is a 8mm plate with a round 105mm hole cutout to take to the upper post, which goes inside the lower square post by 600mm, you can see on the top lower post on the edge of the plate, some concrete, the post were filled with concrete, and and a 16mm reo bar which you can see at the bottom of the round post, , the concrete prevents the steel post from crushing, where the angle brace will be welded to the column, or you could use thicker wall steel, concrete is cheaper and more versatile
You can see where the labour is holding the post 4 x 6mm plates that will be welded on top of the base column, and stops the upper column from going through the base column, as well as giving some extra support to the top post, from bending
